The present invention relates to an anisotropically electroconductive adhesive composition or, more particularly, to an anisotropically electroconductive adhesive composition capable of adhesively bonding two circuit boards between the terminals thereof by a procedure in which a bit of the adhesive is placed between the terminals followed by hot-pressing to produce electric connection between the terminals simultaneously with adhesive bonding.
A diversity of adhesives are known in the prior art for adhesively bonding two electric or electronic circuit boards and establishing electric connection between the terminals thereof at the same time as bonding including those prepared by compounding an electrically conductive particulate material with a thermoplastic polymer such as polyurethanes, polyesters, synthetic rubbers and the like or a heat-curable resin such as epoxy resins, silicone resins and the like.
These conventional conductive adhesives, however, have various disadvantages and are not quite satisfactory. For example, the above named adhesives based on a thermoplastic resin as the matrix may be subject to eventual decrease in the holding power of the base resin or the conductive particles under a high temperature or high humidity condition to cause weakened contact between the terminal and the conductive particles and the electric resistance therebetween is sometimes unstable under such an adverse condition due to microscopic migration of the conductive particles accompanying the flow of the matrix resin to come off the condition of point-contact along with the disadvantages of decrease in the adhesive bonding strength and increase in the electric resistance between the terminals bonded with the adhesive. The adhesives based on the above named heat-curable resins, on the other hand, also have several defects and problems in different respects such as the troublesome procedure of weighing and compounding the component ingredients directly before use for the bonding work since such heat-curable adhesives are mostly supplied in two separate packages each containing the ingredient or ingredients differing from thoss contained in the other package in order to avoid premature setting or curing of the adhesive. The ready-mixed heat-curable adhesive is unavoidably accompanied by a disadvantage of limited length of workable time due to their relatively short pot life. This disadvantage in respect of the limited pot life is also involved even in so-called one package-type adhesives supplied in one package containing all of the necessary ingredients ready-mixed together. Such a ready-mixed adhesive composition is subject to increase, though gradual, of the viscosity or consistency in the lapse of time during storage so that difficulties are encountered in the setting of the working conditions therewith in addition to the problems of decrease in the adhesive bonding strength and increase in the electric resistance between the bonded terminals as well as the poor mass-productivity.